Tunable continuous wave (cw) lasers are essential tools for precision spectroscopy applications such as trace gas detection and frequency measurements. Trace gas detection is particularly sensitive when coupling the cw lasers into a high finesse gas-filled external enhancement cavity which greatly increases the interaction length of the trace gases with the cw light. Because many commercially available cw laser sources, particularly quantum cascade lasers, have relatively large emission linewidths, they cannot be efficiently coupled into high finesse enhancement cavities, which can have acceptance bandwidths as low as 10 kHz, or even lower.
Generally this requirement is resolved by electronic or optical locking techniques. In electronic locking, active feedback components that electronically control the emission wavelength of the cw laser (R. W. P. Dreyer et al., Laser Phase and Frequency Stabilization, Appl. Phys. B, vol. 31, pp. 97-105 (1983)) are provided. Alternatively, electronic locking can also be accomplished by appropriately applying a frequency modulation to the emission wavelength of the cw laser to match the cw laser emission frequency to a resonance frequency of an enhancement cavity (R. Z. Martinez et al., Laser-locked, high-repetition-rate cavity ring-down spectrometer, J. Opt. Soc. Am. B, vol. 23, pp. 727 (2006)).
Optical locking generally uses self-injection of cw laser light as filtered by an external cavity to automatically lock the laser light to the cavity (D. J. Hamilton et al., “A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air”, Appl. Phys. B, vol. 102, pp. 879-890 (2011)). With an optical locking laser, linewidths less than the cavity line width can be obtained.
A well-established method for trace gas detection is cavity ring down spectroscopy, which can be conveniently combined with optical locking techniques as described by Hamilton as well as electronic locking techniques as described by Martinez et al. An extensive review of cavity ring down spectroscopy methods was recently published by A. Cygan et al., Pound-Drever-Hall-locked (PDH), frequency-stabilized cavity ring-down spectrometer, Rev. Scientific Instr., vol. 82, pp. 063107 (2011).